1. Field of the Invention
The present invention relates to a control device for an automatic transmission.
2. Description of the Related Art
In many cases, a vehicle uses an automatic transmission that shifts a power transmission path including a plurality of speed change gear trains and hence automatically changes a speed by selectively performing engagement control for a frictional engagement element such as a speed change clutch. In recent years, a speed change operation of an automatic transmission (an engagement operation of a frictional engagement element) tends to be frequently performed in order to increase power of an engine and to improve quality in speed change of the automatic transmission. Accordingly, in the automatic transmission, a heat quantity of heat that is generated during the engagement operation of the frictional engagement element used for the speed change tends to be increased. In this situation, the frictional engagement element whose temperature is at a high temperature due to the heat generated during the speed change operation is cooled to a temperature equivalent to automatic transmission fluid (ATF) of the automatic transmission mainly through heat exchange with the ATF. Thus, to prevent the fluid temperature of the ATF from being increased is an important factor for the protection of the facing of the frictional engagement element from heat damage such as burnout.
In related art, the fluid temperature of the ATF is detected, and the torque generated by the engine is continuously decreased if the detected fluid temperature is a predetermined temperature or higher. However, a certain time is required for radiation until the heat generated by the frictional engagement element is cooled through the heat exchange with the ATF. A certain difference may be generated between the temperature of the frictional engagement element and the temperature of the ATF. Japanese Unexamined Patent Application Publication No. 2008-101705 (hereinafter, referred to as the document '705) discloses a technique that calculates the temperature of at least a frictional engagement element to be engaged during speed change, and controls a timer such that an upshift timing for the frictional engagement element to be engaged is delayed by a predetermined time if the calculated temperature is higher than a reference temperature. In this case, since the upshift timing is delayed by the predetermined time, the frictional engagement element to be engaged can be cooled, and the heat generation can be suppressed.
Japanese Unexamined Patent Application Publication No. 2006-46569 (hereinafter, referred to as the document '569) discloses a technique that, when a downshift instruction is given, inhibits a downshift operation relating to the downshift instruction if an operation state of an engine is in a predetermined high-load high-rotation region, and if it is judged that a frictional engagement element to be disengaged is at a predetermined high temperature. In this case, by inhibiting the downshift operation if the frictional engagement element is at the predetermined high temperature, the frictional engagement element can be cooled, and the heat generation can be suppressed. The temperature of the frictional engagement element is not actually measured. A timer counts an elapsed time since a previous speed change operation is ended. If the elapsed time is within a predetermined time, it is expected that the frictional engagement element is at the predetermined high temperature.
With the technique disclosed in the document '705, since the temperature of the frictional engagement element is calculated only during the speed change, an increase in temperature during in-gear with a high load is not considered. In addition, a heat quantity term, a temperature increase term, and a cooling term likely depend on turning. The development of these terms may be troublesome work. Also, it is difficult to reliably protect the frictional engagement element unless a timer operation time is set under the most severe condition. A regulation time (a delay time of the upshift timing) has to be long. A driving force provided to a driver may be insufficient due to the response delay. Further, merely the heat generation during the upshift is handled, whereas heat generation during downshift is not considered. Further, since the upshift timing is uniformly delayed at the temperature of a predetermined threshold or higher, the control is rough.
With the technique disclosed in the document '569, it is expected that the temperature of the frictional engagement element is at the predetermined high temperature during the predetermined operation time of the timer. It is difficult to reliably protect the frictional engagement element unless the timer operation time is set under the most severe condition. Thus, the regulation time (the time in which the downshift is inhibited) may be excessively long. A driving force provided to a driver may be insufficient. Also, merely the heat generation during the upshift is handled, whereas heat generation during downshift is not considered.